Brass instruments such as French horns, trombones, tubas, trumpets, and the like, typically include a flared bell connected to a mouthpiece by at least one length of hollow tubing. The tone of the sound produced by the instrument depends, in part, on the length of the tubing connecting the mouthpiece to the bell. Thus, the tone of the sound produced by the musical instrument may be changed by altering the length of the tubing between the bell and the mouthpiece. With the exception of the slide trombone, the addition of tubing length is accomplished by adding loops of tubing that may be selectively activated by use of valves. There are several types of valves used on brass instruments, the most common being piston valves and rotary valves of varying specific designs.
A slide trombone typically includes a mouthpiece that is connected to a bell with tubing having an adjustable sliding section referred to as a handslide. The handslide permits the musician to selectively and continuously vary the length of the connecting tubing while he plays the instrument. To create a tone having lower pitch, the musician increases the length of the tube by pushing the handslide away from himself. Conversely, the musician pulls the handslide back to decrease the length of the tubing creating a higher pitch tone. Musical pieces that require rapid changes between high notes and low notes are more difficult to play because of the required rapid, and accurate, manipulation of the handslide over relatively large distances. To decrease the distances between notes, additional sections of tubing may be added to the instrument and to extend the low register range. The additional loops are selectively incorporated into the active length of tubing by one or more valves, use in conjunction with the handslide.
Rotary valves are among the numerous types of valves used to add and subtract the additional loops into and out of the active length of tubing. Rotary valves generally include a rotor that may be selectively rotated inside a casing. Two or more air ducts are typically disposed in the rotor. The tubes of the instrument are connected to the casing such that the air ducts inside the rotor may selectively form different soundpath configurations for the instrument. A soundpath is defined by the configuration of the tubing of the instrument. One undesirable aspect of known rotary valves is that the cross-section of the tubing does not remain constant through the valve, thus increasing the acoustic impedance of the instrument.
Acoustic impedance degrades the quality of the musical tones produced by the musical instrument by adversely affecting the sound waves as they pass through the instrument. Acoustic impedance generally increases as the length of the tubing used to produce the sound increases. The surface and shape of the tubing also affects acoustic impedance. Generally, the level of acoustic impedance increases as the number of serpentine turns in the tubing increases and as the radii of the turns decrease. Transitions between tubing sections of different cross-sections or diameter are another factor that increases the acoustic impedance. Abrupt or sharp transitions generally have a higher level of impedance than smooth transitions.
One rotary valve disclosing a duct configuration having variable tubing cross-sections is disclosed by U.S. Pat. No. 4,095,504 to Hirsbrunner. Each passageway through the rotor of the valve is generally D-shaped in cross-section while the tubing of the instrument is circular in cross-section. Furthermore, the connections between the rotor ducts and the duct connectors are not smooth and undesirably restrict airflow therethrough. The soundpaths created by the valve exhibit the undesirable sharp transitions, non-constant cross-sections, and tight turns that increase acoustic impedance.
One valve that solves the problems of tight turns and abrupt transitions is disclosed by U.S. Pat. No. 5,361,668 to Andersen et al. The Andersen valve includes a rotor having three passageways therethrough. Two of these passageways are straight while the third is in the shape of a flattened `S`. The Andersen valve affords "straight through" airflow when the valve is in the position where it adds the extra length of tubing. The "straight through" airflow design creates less resistance for the musician while he plays low notes. A problem with the Andersen valve is that the third passageway used to accomplish the "straight through" design causes the overall dimensions of the valve to be too large to be favored by musicians and instrument designers. One limitation of the Andersen valve is that it is specifically designed for a trombone with one or two attachments. Its relatively large physical size makes it too cumbersome to be used on other brass instruments such as French horns, euphoniums, tubas, and the like.
Another problem with some rotary valves is the occurrence of a noticeable sound made as a valve passes back and forth between its actuated and unactuated piston. This unacceptable "air click" is caused by air-flow blockage and concurrent pressure change or release as the rotor passes through the half-valve position. The subsequent sudden pressure release as the ducts align with the external tubing causes an audible "sucking-click" sound ("ffft") which is amplified in the bell of the instrument.
U.S. Pat. No. 5,686,678 to Greenhoe discloses a valve that attempts to alleviate the sudden pressure release as the ducts move from the unactuated to the actuated position. Greenhoe discloses a cylindrical shell rotor with vent openings 51 and 54 machined into the sides of the cylindrical shell. These holes provide some relief from air pressure build up. But, the complete cylindrical shell walls that surround the machined holes impede the vented air flow. Furthermore, Greenhoe discloses a symmetrical 90 degree bend in both passageways. Two 90 degree bends create unwanted impedance of the air flow as it passes through the valve. Furthermore, Greenhoe does not address problems associated with friction that exists between the rotor and the casing.